vetteguy75

Whenever I turn in either air/fuel mixture screws on my Holley Street Avenger 670, I get no reaction from the engine until about 1/2 turn before it "seats"....then it starts to bog down and sputter (like it suppose to). I then turn it back out a total of 1 1/2 turns and it smooths out (both sides have the same reaction). However, I can back the screw out just about all the way out & the engine's rpms stay the same as well as the vacuum... (My vacuum is a steady 12" and idle @ park 850rpms; in drive 650rpms). She runs smooth and strong...no hesitation, no backfire, no problems......

Question is: Isn't the engine suppose to either increase or decrease in speed and vacuum whenever you turn the a/f mixture screws just a little? Don't understand what's going on with mine....if anything.

stingr69

It's a lot easier to see if you have a wideband AF meter. Once you get past the limiting range of the needle when backing out the screws, they loose the ability to modulate the mix.

If you set the primary butterfly so the transition slot resembles a "square" when viewing the carb from the bottom then set the secondary stop to match the amount of light passing through the butterflies on the primary then set the mixture screws to about 1.5 turns out from lightly seated you should be at a very good baseline if the rest of the setup is good. You have to start there and make small adjustments. The screws should respond with some sensitivity of they are doing their job at that point.

-Mark.

vetteguy75

Took the carb off and checked the primary butterfly so the transition slot resembled a "square" & then set the secondary stop to match the amount of light passing through the butterflies on the primary side, re-installed the carb, adjusted the A/F screws....the same situation exists....

SH-60B

If the motor runs good, why are you so concerned with the idle screws?

vetteguy75

I know what you mean...I wanted to make sure I was getting the best A/F....you know, highest possible vacuum, etc etc etc. I'm just confused why the engine doesn't "react" when I turn the A/F mixture screws until just before they seat.....

Any help would be appreciated....

Retro78

Very likely your idle feed restrictors (IFR's) are too large. This is a common symptom. In other words your idle circuit is too rich. The screws do not actually change the amount of fuel running through the circuit. When you get the IFR's in a correct range your idle screws will have an impact and will dial in well between 1 to 1 3/4turns out. Some people put wire in the IFR's on carbs that do not have the screw in type. It is very important to get the idle circuit correct since contrary to popular belief it impacts AFR all the way to 3000 RPM. By running rich you are going through a lot of fuel and reducing the life of your engine significantly. Notice that our modern lean running engines last a lot longer? Whoever originally named the circuit 'idle' have caused many people to incorrectly think it really is just controlling the idle range....very false as anyone who has played with a wide band will tell you. You can change jets all you want, but they will make little difference untill higher RPM's. Read up on how to change the IFR on your carb, hope you have the screw-in type. Although the wire trick does work ok. If you want todo a nice job and really lean how each carb circuit works, buy a wide band.

Bill Curlee

Do you have examples or pictures of adjusting the IFRs (with and without screw in jets??? Im very interested Especially using the wire method. It would make a very informative post!!

Thanks

Bill

WillEE

I just adjusted mine recently and not too different from what you describe. I'm not so sure you really have an issue here. On various engines and carbs that I've done this on it's been pretty close to same reaction. I think you more have a problem when you can't get them to do anything at all but sounds like you are o.k. to me. There numerous methods out there including:

Adusting for max vac.
Adjusting for max RPM
Adjusting till engine stalls and backing off 1/3 turn - I'd verify w/vac. gauge though.

With my current setup which is also Avenger 670 carb on ZZ383 I find that I don't get too much deviation until I get to about the last turn from the screw being fully bottomed out. It all happens within this last turn where I can go from max vac (in my case is only 11.2in.hg due to living at 6000 ft. above sea level) to stalling the engine which happens at about 1/4 turn from being the stop point. So most of my fine tuning occurs between 1/2 & 3/4 turn points. Past that doesn't change things too differently. This has been my experience and I wouldn't be so quick to conclude that you even have a problem going on. I did read the article in recent CarCraft that talks about putting the wires in like person above mentioned and thought it was a good read. But I'm not exactly sure this applies to you since you are seeing adjustment range and able to set it.